Meiosis

• Significance of meiosis in life cycles #1

  Sexual reproduction increases genetic variation because it involves in the combining of genetic material from two (usually) unrelated individuals of the same species, by the process of fertilisation. Genetic variation within a population increase its chances of survival when the environment changes, as some individuals will have characteristics that enable them to be better adapted to the change.
• Significance of meiosis in life cycles #2

  Meiosis means ‘reduction’ and it occurs in diploid germ cells to produce germ cells.The diploid cells undergoing meiosis are in specialised organs called gonads - ovaries and testes. These cells have been in interphase before they enter meiosis.
• Meiosis I

  Prophase 1 :

  starts with a diploid cell

  • its chromatins contain two uncoiled spread out sets of chromosomes one from each parent
  • after DNA in chromatin replicates it condenses into more familiar X shaped chromosomes
• Meiosis 1

  Prophase 1 #2 :

  • synapsis : each chromosomes pairs up with + binds up to its corresponding homologous chromosome

  → { a tetrad , 4 sister chromatids, formed }

  • crossing over - chromatids from each chromosomes exchange segments of alleles so alleles are shuffled

  →random, which is why we’re different from biological parents + siblings

  • nuclear membrane breaks down + centrioles move to opposite ends + spindle fibres fan out from them
• Metaphase I :

  • homologous chromosomes line up at the equator + attach to the spindle fibres from opposite poles
  • homologous pairs are arranged randomly, with members of pair facing opposite poles of cell

  → this arrangement is Independent Assortment

  • the way they line up in metaphase determines how they will segregate independently when pulled apart in anaphase
• Anaphase I :

  • members of each pair of homologous chromosomes are pulled apart by motor proteins that drag along the tubulin threads of spindle
  • centromere do not divide and each chromosome consists of two chromatids
  • the crossed - over areas seperate from each other resulting in swapped areas of chromosomes + allele shuffling
• Meiosis I

  Telophase :

  • each chromosome still consists of 2 sister chromatids

  → no longer identical due to allele exchange in cross-over

  • spindle fibres disappear
  • two new nuclear envelope form around each set of chromosomes + cell divides by cytokinesis
  • each new nucleus contains half the original number of chromosomes, but each chromosome consists of 2 chromatids
  • in most plants, the cell goes straight from anaphase 1 into prophase 2

  result :

  • 2 genetically different haploid daughter cells
• Meiosis II

  Prophase 2 :

  • no DNA replication
  • nuclear envelope disappears
• Meiosis II

  Metaphase :

  • each cell line up at the equator + attach to spindle fibres from both poles
  • centrioles move to opposite ends + spindle fibres fan out from them
• Meiosis II

  Anaphase

  • centromeres divide + chromsome are pulled apart by motor proteins that drag along the tubulin threads of spindle
  • sister chromatids of each chromosome seperate + move to opposite poles, therefore randomly segregated
  • once seperated , they’re called chromosomes
• Meiosis II

  Telophase :

  • spindle fibres disappear
  • nuclear envelopes reform + cytokinesis occurs

  result :

  4 genetically different haploid daughter cells containing only one set of chromosomes
• bivalent 


  homologous chromosomes pairing up
• How meiosis produces genetic variation

  • crossing over during prophase 1 shuffles alleles
  • Independent Assortment of chromosome in anaphase 1 leads to random distribution of maternal + paternal chromosomes of each pair
  • Independent Assortment of chromatids in anaphase 2 leads to further random distribution of genetic material
  • haploid gametes are produced, which can undergo random fusion with gametes derived from another organism of same species